The invention relates to a bias control circuit for a light-emiting diode having compensation of the light output at changing temperature, in which the light-emitting diode (led) per se is used as temperature sensor and is taken up together with a series resistor in a series circuit to which a fixed supply voltage is applied. Such a bias control circuit is known from the German Offenlegungsschrift No. 2,309,446.
When an electric current is passed through a light-emitting diode, this diode will emit a visible or infrared light radiation. The amount of light or light output of the diode depends on the magnitude of the current and on the temperature of the diode. So, the light output will decrease at increasing temperature of the surroundings and vice versa. Also the central wavelength and the band width of the light emitted by the diode will change slightly with temperature. This is, however, less important when the photodetector or photocell, cooperating with a led, which will detect the light, has a sufficiently wide spectral sensitivity.
This dependency on temperature in applications including digital or FM-modulated circuits, is less essential as in these circuits mainly the presence or absence of light or the frequency of the light intensity modulation respectively is detected. However, in applications including analog or proportional circuits the amount of light indeed is essential and the dependency of temperature has to be taken into account.
The bias control circuit of above mentioned Offenlegungsschrift is limited herein, that the resistance value in this circuit is attuned to the light-emitting diode used in accordance with a specific equation. When changing one led for another having mutually different characteristics, the resistance value in this circuit has to be adjusted according to said equation.
In an other bias control circuit known from Netherlands Patent Application No. 80.04071, which has compensation of the light output at changing temperature, apart from a first photodetector cooperating with the led for certain applications, an identical second photodetector as a separate temperature sensor is used. Thereby, besides the amount of light which is transmitted via the object, to be measured, between the led and the first photodetector, also a specified amount of light is transmitted directly to the second photodetector. This second detector together with an amplifier and a control means, connected in series with the diode, is taken up in a control loop to compensate the current through the diode when temperature varies.
The second photodetector required for this known bias control circuit makes this circuit clumsy and even useless, when in certain applications there is only very limited space available for mounting the led and both photodetectors, such as for example in an inflatable cuff having a light plethysmograph.